This invention relates to sound reproduction and, more particularly, to small transducers retained in small housings but having satisfactory frequency response.
The problem of obtaining the best possible frequency response from less than ideal components and structures is as old as the art of sound reproduction. The goal in any system, of course, is to reproduce all of the range of frequencies available in the signal without distortion. In the most expensive audio systems, large cabinets can be used with multiple speakers, elaborately designed baffles, etc., but when there are limitations of size, weight and cost, compromises must be made and sound quality deteriorates. Early in the development, it was found that sound waves passing from the front of a speaker around to the back of the speaker reduced sound quality. A speaker placed in the middle of a wall or a baffle of infinite dimensions was theoretically desirable but not practical. The best practical approach to an infinite baffle was a box or cabinet which is closed or nearly closed in the back. In a typical arrangement, the resonance of an 8" speaker in a closed box would likely be in the range of 60-200 Hz, with output decreasing at 12 db/octave below resonance. Resonance would be in the order of 30-60 Hz for 12-15" speakers. Techniques have been developed which can extend the normal response another octave down, with subsequent falloff at perhaps 24-30 db/octave. It has not been considered a desirable solution to move the resonant frequency higher than the desired frequency range.
In a U.S. Pat. No. 4,481,662, processing of the signals being input to a speaker was used to flatten out the frequencies below the system resonance, with the result that frequencies above normal resonance were attenuated by 12, 18 or 24 db/octave. Frequencies above cutoff were attenuated by another 12 db/octave.
In telephones and telephone-type devices, the speaker is small with close coupling to the ear of the user, and can provide a reasonably flat response from 250 to 3000 Hz. In the case of portable phones and radios, however, the speaker and speaker enclosure are likely to be much smaller and thinner, and telephone sound quality is very difficult to obtain.